During the drilling and completion of oil and gas wells, it may be necessary to engage in ancillary operations, such as evaluating the production capabilities of formations intersected by the wellbore. For example, after a well or well interval has been drilled, zones of interest are often tested to determine various formation properties such as permeability, fluid type, fluid quality, formation temperature, formation pressure, bubblepoint and formation pressure gradient. These tests are performed in order to determine whether commercial exploitation of the intersected formations is viable and how to optimize production. The acquisition of accurate data from the wellbore is critical to the optimization of hydrocarbon wells. This wellbore data can be used to determine the location and quality of hydrocarbon reserves, whether the reserves can be produced through the wellbore, and for well control during drilling operations.
A downhole tool is used to acquire and test a sample of fluid from the formation. More particularly, a probe assembly is used for engaging the borehole wall and acquiring the formation fluid samples. The probe assembly may include an isolation pad to engage the borehole wall. The isolation pad seals against the formation and around a hollow sample probe, creating a sealing arrangement that creates a seal between the sample probe and the formation in order to isolate the probe from wellbore fluids. The sealed probe arrangement also places an internal cavity of the tool in fluid communication with the formation. This creates a fluid pathway that allows formation fluid to flow between the formation and the formation tester while isolated from the borehole fluids. The fluid pathway may be enhanced by extending the sample probe to couple to the formation.
In order to acquire a useful sample, the probe must stay isolated from the relative high pressure of the borehole fluid. Therefore, the integrity of the seal that is formed by the isolation pad is critical to the performance of the tool. If the borehole fluid is allowed to leak into the collected formation fluids, a non-representative sample will be obtained and the test will have to be repeated.
Formation testing tools may be used in conjunction with wireline logging operations or as a component of a logging-while-drilling (LWD) or measurement-while-drilling (MWD) package. In wireline logging operations, the drill string is removed from the wellbore and measurement tools are lowered into the wellbore using a heavy cable (wireline) that includes wires for providing power and control from the surface. In LWD and MWD operations, the measurement tools are integrated into the drill string and are ordinarily powered by batteries and controlled by either on-board or remote control systems. With LWD/MWD testers, the testing equipment is subject to harsh conditions in the wellbore during the drilling process that can damage and degrade the formation testing equipment before and during the testing process. These harsh conditions include vibration and torque from the drill bit, exposure to drilling mud, drilled cuttings, and formation fluids, hydraulic forces of the circulating drilling mud, high downhole temperatures, and scraping of the formation testing equipment against the sides of the wellbore. Sensitive electronics and sensors must be robust enough to withstand the pressures and temperatures, and especially the extreme vibration and shock conditions of the drilling environment, yet maintain accuracy, repeatability, and reliability.
A generic formation tester is lowered to a desired depth within a wellbore. The wellbore is filled with mud, and the wall of the wellbore is coated with a mudcake. Once the formation tester is at the desired depth, it is set in place and an isolation pad is extended to engage the mudcake. The isolation pad seals against mudcake and around the hollow sample probe, which places an internal cavity in fluid communication with the formation. This creates the fluid pathway that allows formation fluid to flow between the formation and the formation tester while isolated from wellbore fluids.
The isolation or seal pad is generally a simple rubber pad affixed to a metal support member. The outer sealing surface is cylindrical or spherical. Stresses from use and downhole pressures and temperatures tend to quickly fatigue the rubber pad, leading to premature failure. Therefore, there remains a need to develop an isolation or seal pad that provides reliable sealing performance with an increased durability and resistance to stress. In this manner, an extended seal pad life provides an increased number of tests that can be performed without replacing the pad.